Other Species

Killer shrimp          

Dikerogammarus villosus

 Identification sheet

A very recent introduction, first reported in the UK in 2010 and since having been found in three different locations. This is currently a high priority for biosecurity, the emphasis of management being on stopping any further spread. An alert species, it must be reported as soon as possible on being discovered. The killer shrimp has been ranked among the ‘Top 100’ invasive alien species in Europe by an EC initiative. In Britain the Environment Agency carry out extensive monitoring of water bodies for this species.

The risk assessment for killer shrimp has scored this species ‘very high risk’ for impact and ‘high’ risk overall. It is a voracious predator of native shrimp, other invertebrates, eggs and small fish. Through direct predation and indirect trophic effects, it has the potential to significantly alter ecosystems.

You can help stop the spread of killer shrimp and other aquatic invasives by following three simple steps whenever you visit freshwaters: Check – Clean – Dry.

For more biosecurity advice, please visit www.nonnativespecies.org/alerts/killershrimp

Zebra mussel

Dreissena polymorpha

 Identification sheet

Like killer shrimp, the zebra mussel is an import from the Ponto-Caspian region of Western Asia. However it has been in Britain much longer, since the early 19th century.  It is present in Lancashire but not widespread. Zebra mussels are so named because of the distinctive zigzag banding on their shells. They out-compete native fauna, forming dense colonies on the substrate under still or flowing water. They also filter large quantities of water, removing planktonic animals which are the food of many other species. 

Chinese mitten crab          

Eriocheir sinensis

 Identification sheet

This large riverine crab gets its name from the velvety hair that covers its pincers. Transported to Britain in ballast water, the mitten crab was first recorded in the river Thames. It has now been found in several locations includingSouth Cumbria. This is an alert species so please report immediately if seen. Mitten crabs are voracious predators, eating a variety of native wildlife. They also riddle the river bank with burrows, causing erosion and subsidence.

Canadian and Nuttall’s pondweeds

Elodea species

 Identification sheet

These pondweeds are true aquatic plants which grow in deep, slow flowing water. Dispersal is by vegetative means – small fronds of plant break off and float away to settle elsewhere. The impact of these plants is not well known but they can form dense beds which exclude other aquatic plants and may impede stream flow in some circumstances.


Curly water thyme

Lagarosiphon major 

 Identification sheet

This waterweed superficially resembles the Elodea species (Canadian and Nuttall’s pondweeds), however it is more invasive. It is native to Southern Africa and inhabits deep, still or slow moving water. All plants in Britain are female and it is spread by the movement of plant fragments. Like other troublesome aquatic invasives, this species impacts on waterways by growing dense colonies and excluding native plants.

Floating pennywort

Hydrocotyle ranunculoides

 Identification sheet

This plant is a recent introduction from the Americas. It spreads vigorously across water and damp soil, preventing light reaching the water and excluding other plants. It also interferes with recreational use of the water. At the moment, floating pennywort is not widespread in the North West. It is important to monitor this plant closely to prevent further spread.


Creeping water primrose          

Ludwigia peploides

 Identification sheet

This is a worrisome plant that is fortunately not yet present in Lancashire. It is native throughout the Americas and was first recorded in Britain in 1999. This is an alert species which must be reported as soon as possible to the Non-Native Species Secretariat. Creeping water primrose forms large monoculture stands, shading out other plants, disrupting water flow and increasing the flood risk. It has an incredible potential for growth and can double its biomass in 20 days.

Parrot’s feather

Myriophyllum aquaticum

 Identification sheet

Named after its feathery appearance, this plant originates from South America. The plant grows submerged in the water and may crowd out other aquatic plants due to its vigorous habit. Its habitat tends to be still water: ponds, ditches and canals, but it is occasionally found in slow flowing rivers. Like floating pennywort, parrot’s feather is not well established in Lancashire.


Water fern

Azolla filiculoides

 Identification sheet

Azolla is the only floating fern found in Britain. It is often spread inadvertently on aquatic garden plants and is sometimes even sold as an ornamental. From just a small fragment, the fern will quickly spread across the surface of the water, forming a dense mat and completely excluding light penetration. The roots of the plant form a symbiosis with a nitrogen fixing cyanobacteria, enabling azolla to grow in nutrient poor water.


Australian swamp stonecrop

Crassula helmsii

 Identification sheet

Also known as New Zealand pigmyweed, this succulent perennial is one of the more problematic of the invasive aquatic plants. Its habitat is ponds, canals, wetlands and other areas of still water. It has a creeping habit and covers the surface of the water to a depth of half a metre. Dispersal in Britain is primarily vegetative; stonecrop can regenerate from a single node on a 10mm stem fragment.

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